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Energy system structure and power flow analysis of pure electric vehicle

Energy system structure and power flow analysis of pure electric vehicle

The power battery energy management system is one of the key technologies of electric vehicles. At present, the research on power battery energy management system mainly focuses on the energy management strategies of hybrid electric vehicles and pure electric vehicles. Due to the complex powertrain system of hybrid electric vehicles, there are many control strategies and a large space for development. For example, the commonly used control strategies for series hybrid electric vehicles include thermostat strategy, power tracking strategy and basic rule strategy; the commonly used control strategies for parallel hybrid electric vehicles include static logic threshold strategy, instantaneous optimal energy management strategy, and fuzzy logic control. strategy and global optimal energy management strategy, etc.; the commonly used control strategies for hybrid hybrid vehicles include engine constant operating point strategy, engine optimal working curve strategy, etc.

Pure electric vehicles can be divided into multiple energy source systems and single energy source systems according to the number of energy sources. The multi-energy source is mainly a dual-energy source system composed of a battery and a supercapacitor. The main feature of the supercapacitor’s large charge and discharge rate is used to make up for the shortcomings of the power battery by cutting peaks and filling valleys. At the same time, due to the existence of supercapacitors, which increases the complexity of the powertrain system, the available control strategies are also much more than that of single energy sources, such as threshold control strategies and fuzzy logic control strategies. For pure electric vehicles with a single energy source, because the powertrain system is simpler than that of hybrid and multi-energy source systems, there is less room for control strategies.

Combining the energy management and control strategies of pure electric vehicles of Chinese and foreign OEMs, the main control strategies are as follows: one is to reduce the energy usage rate of the entire vehicle, and only retain the high-voltage load necessary for the vehicle to travel, so as to minimize the energy consumption of the entire vehicle. The power consumption of the high-voltage system is to allocate all the limited power to the drive system; the second is to improve the efficiency of battery use. Through the use of the thermal management system, the battery has been controlled in the high-efficiency range. Although the first method can save the power consumption during driving to the greatest extent, due to the lack of the battery thermal management system, the long-term high temperature operation will accelerate the aging of the battery, which will sacrifice the economic performance of the vehicle; the second method is the most commonly used threshold. Although the control strategy is simple and stable, it cannot optimally solve the power distribution problem due to the fixed control rules, thus affecting the dynamic performance of the vehicle.

  1. Energy system structure and power flow analysis of pure electric vehicle

The high-voltage system components of pure electric vehicles are mainly divided into air-conditioning system components, drive system components, low-voltage power supply system components and charging system components. Among them, the low-voltage power supply system components convert high-voltage electricity into low-voltage electricity to support the entire vehicle electronic components , the operation of low-voltage equipment such as power steering, water pumps and fans. Charging system components replenish energy from the grid through chargers or other charging equipment. The air conditioning system components are mainly used to improve driver comfort and thermal management of the power battery. The high-voltage components are the electric heater (PTC) for heating and the air compressor (ACP) for cooling, and the drive system components are used to drive the motor to the outside. Doing work, you can also brake to recover part of the energy.

From the above-mentioned components of the power battery system and the high-voltage system, a schematic diagram of the power flow of the vehicle energy management system as shown in FIG. 1 can be obtained. It describes the input-output relationship of power flow between the power battery system and the high-voltage system of the vehicle.

Figure 1 - Power flow of vehicle energy management system
Figure 1 – Power flow of vehicle energy management system

In the figure, Pbat represents the maximum output power of the power battery system. PA-c represents the maximum power allocated by the energy management system to the air conditioning system. Since the comfort system and the thermal management system share power devices, in order to describe their functions intuitively, the air conditioning system is equivalent to a thermal management system, which is called PT-M. Pmotor represents the maximum power that the energy management system can allocate to the drive system. PL-p represents the required power of the low-voltage power supply system. Pm represents the maximum rechargeable power of the charging system.
During the driving process of the vehicle: Pcha(t)=0; PL-p(t) is a fixed value, which is equal to the DC/DC rated power in value and must be allocated; PT-M(t) and Pmotor(t) The size can be determined according to the power required by the power bus under different working conditions, and it is an amount that can be adjusted and allocated. Let the power that can be allocated by the vehicle energy management system be Pe(t), then:

Obviously, the purpose of the vehicle energy management system is to reasonably allocate Pe(t) to PT-M(t) and Pmotor(t), so as to maximize the energy utilization efficiency of the vehicle.

Read more: What is a Lithium-Ion Power Battery Pack

Power battery-battery energy management system hardware slave board design

Power battery-battery energy management system hardware slave board design

  1. System framework and overview

The battery management system mainly implements the collection and reporting of voltage and temperature signals on a single battery module from the board, and performs balancing operations on the cells on the module when the balancing function is executed.
Multiple battery modules are used in the pack system design, and each battery module uses a slave board. The slave board is mainly used for the collection, reporting and equalization functions of battery cell voltage and temperature. The slave board hardware generally includes a dedicated battery acquisition chip, an isolation chip, a single-chip microcomputer, and a communication circuit. The block diagram of the slave board system is shown in Figure 1.

Figure 1 Block diagram of the slave board system
Figure 1 Block diagram of the slave board system

The MCU main control unit is placed on the high-voltage battery side, only the CAN module is placed on the low-voltage side of the slave board, and a power chip is placed on the high-voltage side and the low-voltage side, which can reduce the isolation of one power supply and reduce the EMC problem of the entire board. The functional requirements of the slave board system are as follows:
(1) Slave board channel: Each slave board has 4 voltage acquisition channels and is compatible with 5-channel mode, and 5 temperature acquisition channels. Only 4 channels can be selected as the acquisition interface.
(2) Acquisition accuracy: single voltage accuracy ± 5mV, measurement range 0~5V, temperature measurement range -40°C~85°C, required accuracy at -30°C~60°C ≤±1°C, 60°C~85°C required Accuracy≤±1.5℃.
(3) Acquisition time: the single voltage reporting period is 50ms, and the temperature reporting period is 50ms.
(4) Communication mode: The communication between the slave board and the main board adopts high-speed fault-tolerant CAN, with a rate of 500kbit/s.
(5) Acquisition method: special acquisition chips are required to simplify system design and ensure scalability.
(6) Power supply mode: the power supply current of the high-voltage side does not exceed 50mA, and the power supply current of the low-voltage side does not exceed 30mA.

  1. Processor and chip and power supply

2.1. Processor
The slave board processor is used to monitor, process and report the battery voltage and temperature signals, and the functions can be realized by using the single chip microcomputer. The processor uses the freescale chip MC9S08DZ60. The processor must include at least ROM, RAM, and flash storage space, of which the EEPROM requirement is not less than 1K, the RAM requirement is not less than 2K, and the flash requirement is not less than 32K. Hardware watchdog function: The processor power-on completion time is required to be within 1s, and it can wake up and sleep through hard wires; the processor has the CAN interface function to communicate with the motherboard, and the SPI or I2C interface function to communicate with the internal chip.

2.2. Acquisition chip and isolation chip
The acquisition chip is used to manage the acquisition function of battery voltage and temperature. The function can be realized by using a special acquisition IC. The LTC6804HG-1 acquisition chip of Linear Technology is selected, which can provide the following resources: the single voltage acquisition channel is 12channel, 5 GPI0 ports (can be multiplexed into 5 temperature acquisition channels); multiple acquisition chips are allowed to be used in parallel, and a daisy-chain structure can be provided. The acquisition resolution of the voltage channel is 16bit, the total voltage acquisition accuracy (including analog front-end and back-end processing) meets -2.8~2.8mV, the acquisition time of the acquisition chip is 130us, the withstand voltage requirement of the acquisition chip is not less than 60V, and the voltage acquisition channel The measurement range is 0~5V, and the acquisition chip with hardware diagnosis function channel can perform hardware diagnosis on the undervoltage and overvoltage of the battery cell voltage, and report the fault status. The acquisition chip has the SPI interface function to communicate with the processor, and the chip temperature range is -40℃~125℃. The isolation chip is required to meet the electrical isolation of the battery side and the low-voltage side, and the electrical isolation meets the requirements of insulation and safety regulations. The electrical design defines the RMS voltage of 400V, and the minimum clearance and creepage distance of 4.00mm, which are mainly considered in the PCB layout. The isolation chip uses ADI’s I Coupler digital isolation chip ADuM12011.

2.3. Power supply
The slave board is powered by the low-voltage system and the high-voltage system, and the power supply system is designed as follows: the low-voltage power supply comes from the low-voltage battery of the whole vehicle (about the knowledge of the battery, I accidentally found an article before, and found that the author knows the knowledge of the battery Very thorough, if you are also interested, you can visit Tycorun Battery to read)
), the normal working voltage is 12V, the voltage range is 6~16V, and the working current does not exceed 50mA. Use the power management chip for power supply control, provide internal 5V or 3.3V, and supply power for CAN and isolation chips. The high-voltage power supply comes from the module of the high-voltage battery, and the voltage range is 8~25V. The high-voltage module directly provides power for the acquisition chip, and converts it into 5V through DC-DC or LDO to power the MCU and other chips on the high-voltage side. Power management can support power-on and power-off management of hard-wired Enable. Enable high level wakes up the power management chip and performs power-on initialization. It is required to complete initialization and start measurement within 120ms, and send a normal CAN signal; after Enable low level, the power management has a self-locking function, which supports the processor after the power-off management is completed. , go to sleep again.

  1. Interface definition and CAN communication

3.1. Interface Definition
The slave board is respectively connected to the battery terminal and the main board. The design requirements for the interface are as follows: the voltage input interface channel of the battery terminal is 4 channels, the temperature input interface channel is 4 channels, the connection terminals are designed separately, and the on-board connection terminals are used. The power supply terminal and the main board communication terminal have Redundant design to ensure the cascading of multiple acquisition sub-boards. The cascading method is shown in Figure 2.

Figure 2 Slave board cascade mode
Figure 2 Slave board cascade mode

3.2.CAN communication
The slave board has communication functions such as CAN. The communication follows the following requirements: the external port of the slave board needs to provide at least one high-speed CAN communication with the main board, the communication rate is 500kbit/s, and the CAN2.0 communication protocol. The acquisition chip and processor need to have communication functions such as SPI or I2C for internal communication; CAN needs to reserve a terminal resistance, and the CAN network ID can be configured independently. The voltage and temperature signals are collected from the board and reported to the main board through the CAN signal. At the same time, the equalization command of the main board is sent to the slave board through the CAN signal to realize the equalization function.

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